Micromechanical Testing Research Articles

Twin Boundary Sliding in Single Crystalline Cu and Al Nanowires

Twin boundary sliding (TBS) is a deformation mode that is typically observed in nanocrystalline metals and usually observed during the final deformation stage when deformation via twinning is limited. Unlike the well-established dislocation slip and necking deformation process that occurs in large crystals, recent theories that explain TBS are often controversial, and much remains unsettled. Herein, we develop a theory that enables the prediction of deformation pathways by quantitatively analyzing the relative tendency for the formation of partial dislocations based on the dislocation and fault energy theories. The developed theory considers the effects of static features, such as the stacking fault energy (SFE) as well as the crystal size and orientation, and dynamic structural states characterized by the various fault energies of a material. The theory is initially validated using a Cu crystal exhibiting low SFE for various orientations and sizes. In addition, micro-mechanical tensile tests and molecular dynamics simulations are conducted on Al nanowires to determine whether the proposed theory can be generically extended to crystals with high SFE. The developed theory produces self-consistent results even for metals exhibiting different SFE values. The observations can be used as a guideline to design nanoscale structures for load-carrying applications.

Assembly of a simple scalable device for micromechanical testing of plant tissues.

Tensile testing is widely used to evaluate the mechanical properties of biological materials including soft primary plant tissues. Commercially available platforms for tensile testing are often expensive and limited in customizability. In this chapter, we provide a guide for the assembly and use of a simple and low-cost micromechanical testing apparatus suitable for research and educational purposes. The build of the setup is presented with scalability and universality in mind and is based on a do-it-yourself mind frame towards mechanical tests on plant organs and tissues. We discuss hardware and software requirements with practical details on required components, device calibration and a script to run the device. Further, we provide an example in which the device was used for the uniaxial tensile test of onion epidermis.

Development and optimisation of micromechanical testing techniques to study the properties of meniscal tissue

In this paper we present the results from a recent micromechanical investigation aimed at developing methodologies for testing and understanding the fundamental behaviour of meniscal tissue. To achieve this, we employed two distinctly different, but equally relevant mechanical testing platforms – uniaxial tensile testing and Dynamic Mechanical Analysis. The results from the tensile tests revealed that the studied material exhibits non-linear stress–strain behaviour and that its viscoelastic properties are time-dependent. Furthermore, by using DMA it was possible to perform walking and running simulations, which provided further information of the strain = time response of the meniscal samples. The importance of accurate specimen preparation and actual method development are also presented and discussed in detail.

Low-temperature rheology of calcite

SUMMARY Low-temperature plastic rheology of calcite plays a significant role in the dynamics of Earth's crust. However, it is technically challenging to study plastic rheology at low temperatures because of the high confining pressures required to inhibit fracturing. Micromechanical tests, such as nanoindentation and micropillar compression, can provide insight into plastic rheology under these conditions because, due to the small scale, plastic deformation can be achieved at low temperatures without the need for secondary confinement. In this study, nanoindentation and micropillar compression experiments were performed on oriented grains within a polycrystalline sample of Carrara marble at temperatures ranging from 23 to 175 °C, using a nanoindenter. Indentation hardness is acquired directly from nanoindentation experiments. These data are then used to calculate yield stress as a function of temperature using numerical approaches that model the stress state under the indenter. Indentation data are complemented by uniaxial micropillar compression experiments. Cylindrical micropillars ∼1 and ∼3 μm in diameter were fabricated using a focused ion beam-based micromachining technique. Yield stress in micropillar experiments is determined directly from the applied load and micropillar dimensions. Mechanical data are fit to constitutive flow laws for low-temperature plasticity and compared to extrapolations of similar flow laws from high-temperature experiments. This study also considered the effects of crystallographic orientation on yield stress in calcite. Although there is a clear orientation dependence to plastic yielding, this effect is relatively small in comparison to the influence of temperature.

Micromechanical testing and modelling of blast furnace slag cement pastes

This work aims to understand deformation and fracture processes in blast furnace slag cement pastes made using CEM III/B which is commonly used in the Dutch infrastructure sector. First, based on our previous work on Portland cement pastes, a micromechanical model utilizing nanoindentation and X-ray computed tomography (CT) for input is created. Statistical analysis are carried out and shows that grayscale values from X-ray CT scans of slag pastes can be linearly correlated with nanoindentation measurements of elastic modulus. Simulations of uniaxial tension are then performed for varying w/c ratios using the Delft lattice model and microstructure obtained from X-ray CT. In addition, advanced micromechanical experiments for estimating the micro-scale tensile strength and elastic modulus are performed. Experimental and simulation results are then critically discussed and compared. It shows that simulation results match the measured tensile strength quite well although some discrepancy does exist at lower w/c ratios. In addition, the observations are compared to our previous findings on ordinary Portland cement pastes. It is found that tensile strength and elastic moduli of slag pastes at 28 days are higher than those of Portland cement pastes with the same w/c ratio. This study will form a basis for micromechanical testing and modelling of blended cement paste systems in the future.

Structure of windmill palm (Trachycarpus fortunei) culm and its compressive properties

Windmill palm (Trachycarpus fortunei (Hook.) H. Wendl.) is a typical vascular plant containing parenchyma cells and vascular bundles. The study of palm vascular bundles and their mechanical properties is of primary importance for a better understanding of the mechanical properties of the culm of windmill palm. Various microscopic techniques were used to observe and quantify the variation in vascular bundles along the radial direction, and micromechanical testing machines were used for the determination of mechanical properties of windmill palm culm blocks and pure parenchyma cells. It was observed that the proportional area of vascular tissue and parenchyma cells increased markedly with the decrease in the proportional area of fibrous sheaths and the number density of vascular bundles along the radial direction. The same trend was also observed for the shape of the vascular bundles changing from oval to round. From the inner to the outer layers, the value of the apparent Young’s modulus and compressive strength continuously increased with increase in fibrous sheath volume fraction, varying from 0.54 to 1.07 GPa, and 26.83 to 69.33 MPa, respectively. The theoretical average value of the compressive Young’s modulus of the fibrous sheath was 2.74 GPa, which is similar to that of the windmill palm leaf sheath fiber. These results provide a better understanding of the compressive performance of windmill palm culm associated with its hierarchical structure.

The effect of strain rate and orientation on He ion irradiated Ni single crystals – An in situ micro-tensile study

The strain rate response of He2+ ion-irradiated Ni single crystal samples along <100> and <110> crystallographic directions is studied here under two different irradiation conditions at room temperature. For one set of samples, a single energy (SE) He2+ ion beam of 5 MeV was used to irradiate the foil to a fluence of 1.7 × 1017 ions/cm2 while the other set of samples were irradiated to 4.4 × 1017.ions/cm2 using an energy degrader wheel, thus creating an approximately uniform dispersion of energies (DE). Micro-tensile samples were fabricated from the irradiated specimens via Focused Ion Beam (FIB) milling and tested at nominal displacement rates of 5 nm/s and 500 nm/s. These corresponded to nominal strain rates of the order of 1.7 × 10−4 and 1.7 × 10−2/s, respectively. Electron back scatter diffraction (EBSD) analysis was performed on the samples pre- and post-tensile testing to elucidate the deformation behaviour. A significant finding of this study is that the strain rate sensitivity of the samples depended strongly on the dose and distribution of the radiation damage, and that irradiation caused a suppression of the strain rate sensitivity. The degree of strengthening and embrittlement is particularly pronounced for the DE irradiated samples as its strength increased fourfold and its plasticity was reduced to almost zero. The critical resolved shear stress (CRSS) was derived for different irradiation conditions and a linear relationship was determined between the CRSS and the irradiation damage experienced. The significance of the results in comparison to alternative micro-mechanical testing methods as well as the effect of irradiation on different crystal orientations is further discussed in this work.

Fatigue Behavior of SAC-Bi and SAC305 Solder Joints With Aging

Reliability of microelectronic assemblies is typically limited by the fatigue failure of one of the interconnected solder joints. The fatigue behavior of the lead-free solder joints doped with bismuth under different stress loading and aging conditions is not yet understood. This article investigates the effect of adding bismuth on the mechanical reliability of SAC alloys considering different loading and aging conditions. The fatigue behavior and shear strength of individual SAC305 (Sn–3.0Ag–0.5Cu) and SAC-Q (Sn–3.4Ag–0.5Cu–3.3Bi) solder joints are examined and compared. A unique experiment is designed to test the individual solder joints using a micromechanical testing system. The results show that the fatigue life and shear strength of SAC-Q with Bi are much higher than SAC305 regardless of the aging and stress conditions. It was also found that increasing stress amplitude leads to a decrease in the fatigue life for both alloys. The aging time has a negative effect on the fatigue life and shear strength of both alloys. The impact of aging on SAC-Q solder joints is significantly less than that for SAC305. Microstructure analysis shows a substantial amount of precipitates coarsening with aging for SAC305 compared with SAC-Q. Hysteresis loop analysis shows that increasing the cycling stress amplitude and aging time leads to an increase in the work per cycle and plastic strain. Common fatigue models, such as Morrow energy model and Coffin–Manson model, fail to predict the life considering the effect of aging; aging affects the constants of both models.

A micromechanical study of shale rock-proppant composite interface

The micromechanical properties of shale rock-sand type of composite interface were investigated by performing laboratory tests with a custom-built loading apparatus. This type of interface is of significant interest in petroleum engineering in the process of hydraulic fracturing using proppants. Two different types of quartz sands of different particle shapes were used as proppant simulants and they were sheared against rough and smooth surfaces of shale. It was shown that sand grain type and surface roughness of shale affected the contact stiffness in the normal and tangential directions because of their influence on the induced contact stresses. Based on the micromechanical shearing tests, it was found that the coefficient of friction, the initial tangential stiffness and the microslip displacement were positively correlated with the magnitude of normal load. Although clear surface damage/plowing of the shale surface caused by shearing was observed and captured by interferometry images, the effect of loading history was found to be small. Shearing tests on wet and dry shale surfaces did not show that much difference in results, but immersion of shale in water prior to shearing affected the initial tangential stiffness and coefficient of friction at certain normal loads. Smoothening of the surface had an influence on friction as long as limited plowing was triggered in the low range of normal loads, so that the influence of surface roughness of the shale would diminish at higher normal loads where plowing mechanism dominated the interface response.

Physicochemical characterization and antimicrobial activity of edible propolis-chitosan nanoparticle films

The aim of this study was to evaluate the physicochemical properties and antimicrobial activity of edible propolis-chitosan nanoparticle films. Five formulations containing chitosan (C1), chitosan nanoparticles (C2) and ethanolic extract of propolis (EEP) at different concentrations (C3 = 10%, C4 = 20% and C5 = 30%) were prepared. From the results, a particle size of 28.42 ± 7.43 nm was observed from transmission electron microscopy (TEM) micrographs for chitosan nanoparticles (CSNPs). Contact angle measurements showed a more hydrophilic (0°) behavior for C1 and C4. On the other hand, the most hydrophobic behavior was observed for C3 and C5, with values of 21.38° and 21.17°, respectively. Optical micrographs of coated strawberries showed good adhesion with thickness values between 146.15 and 341.88 μm. No significant differences were found for moisture content, hardness and X-ray diffraction (XRD) (p < 0.05). When nanoparticles were incorporated into the formulations and propolis content increased, the degree of swelling and water solubility decreased. Fourier-transform infrared spectroscopy (FTIR) spectra revealed possible interactions among components. From the micromechanical tests, the highest value of elastic modulus (2.6 GPa) was obtained for C3. From AFM studies, the addition of chitosan nanoparticles and propolis decreased the value of roughness. The antibacterial activity evaluation showed that after 24 h, the propolis-containing formulations (C3-C5) had a strong inhibitory effect on Listeria monocytogenes and C3 on Escherichia coli. The use of chitosan nanoparticles and ethanolic extract of propolis in edible films is a promising alternative for active packaging with antibacterial properties in view of their application for the preservation of strawberries against foodborne bacteria.

Microscale fracture of chromia scales

Native protective oxide scales offer resistance against corrosion for high temperature materials, which often work in extreme conditions of varying mechanical and thermal loads. The integrity of such layers is of critical importance, since their damage can lead to significant reduction in material life. Mechanical data such as fracture strain and elastic modulus are required to include oxides in material life estimation models for high temperature materials, but there is lack of such data. Their thickness is in the μm range, which makes mechanical testing for property determination difficult. Here we present a micro-mechanical testing method, based on bending of micro-cantilevers produced by focused ion beam milling, capable of circumventing the limitations of conventional approaches. We apply this method to chromia thermally grown on pure chromium, and measure fracture strains at room and high temperatures (600 °C). The measured fracture strains were found to be higher at room temperature, due to a larger fraction of transgranular fracture. Surprisingly, a large fraction of transgranular fracture was seen even in the presence of stress concentrations at grain boundaries. Removal of the stress concentrations accentuated the propensity for transgranular cracking at room temperature. Realistic values of room temperature elastic modulus were obtained as well. The observed mixed trans- and intergranular cracking points towards the need for dedicated investigations of both oxide grain boundary strength and cleavage resistance of single crystals in order to fully understand the failure mechanisms in thermally grown oxide scales.

One‐step deposition of nano‐to‐micron‐scalable, high‐quality digital image correlation patterns for high‐strain in‐situ multi‐microscopy testing

AbstractDigital image correlation (DIC) is of vital importance in the field of experimental mechanics, yet producing suitable DIC patterns for demanding in‐situ (micro)mechanical tests remains challenging, especially for ultrafine patterns, despite the large number of patterning techniques reported in the literature. Therefore, we propose a simple, flexible, one‐step technique (only requiring a conventional physical vapour deposition machine) to obtain scalable, high‐quality, robust DIC patterns, suitable for a range of microscopic techniques, by deposition of a low‐melting temperature solder alloy in the so‐called island growth mode, without elevating the substrate temperature. Proof of principle is shown by (near‐)room temperature deposition of InSn patterns, yielding highly dense, homogeneous DIC patterns over large areas with a feature size that can be tuned from as small as ~10 nm to ~2 μm and with control over the feature shape and density by changing the deposition parameters. Pattern optimisation, in terms of feature size, density, and contrast, is demonstrated for imaging with atomic force microscopy, scanning electron microscopy, optical profilometry, and optical microscopy. Moreover, the performance of the InSn DIC patterns and their robustness to large deformations is validated in two challenging case studies of in‐situ micromechanical testing: (a) self‐adaptive isogeometric digital height correlation of optical surface height profiles of a coarse, bimodal InSn pattern providing microscopic 3D deformation fields (illustrated for delamination of Al stretchable interconnects on a PI substrate) and (b) DIC on scanning electron microscopy images of a much finer InSn pattern allowing quantification of high strains near fracture locations (illustrated for rupture of a polycrystalline Fe foil). As such, the high controllability, performance, and scalability of the DIC patterns, created by island growth of a solder alloy, offer a promising step towards more routine DIC‐based in‐situ micromechanical testing.

Micromechanical Behavior of DNA‐1A Lunar Regolith Simulant in Comparison to Ottawa Sand

AbstractIn this study, the micromechanical interparticle contact behavior of “De NoArtri” (DNA‐1A) grains is investigated, which is a lunar regolith simulant, using a custom‐built micromechanical loading apparatus, and the results on the DNA‐1A are compared with Ottawa sand which is a standard quartz soil. Material characterization is performed through several techniques. Based on microhardness intender and surface profiler analyses, it was found that the DNA‐1A grains had lower values of hardness and higher values of surface roughness compared to Ottawa sand grains. In normal contact micromechanical tests, the results showed that the DNA‐1A had softer behavior compared with Ottawa sand grains and that cumulative plastic displacements were observed for the DNA‐1A simulant during cyclic compression, whereas for Ottawa sand grains elastic displacements were dominant in the cyclic sequences. In tangential contact micromechanical tests, it was shown that the interparticle friction values of DNA‐1A were much greater than that of Ottawa sand grains, which was attributed to the softer contact response and greater roughness of the DNA‐1A grains. Widely used theoretical models both in normal and tangential directions were fitted to the experimental data to obtain representative parameters, which can be useful as input in numerical analyses which use the discrete element method.

Microscale assessment of corneal viscoelastic properties under physiological pressures

The micromechanical behaviour of the cornea is important for understanding and modelling of many ocular disorders. Whereas inflation testing has been utilised to determine the bulk mechanical properties of the cornea under physiological pressures, micromechanical testing has been limited to unpressurised corneal samples. In this study the micromechanical properties of pressurised porcine corneas were determined using oscillatory nanoindentation coupled with a custom inflation method. Inflation was conducted in phosphate buffered saline (PBS) and tissue culture (TC) solutions. The shear storage modulus (G′) and shear loss modulus (G”) was determined for corneas inflated corneas with PBS and TC. Central corneal thickness (CCT) was monitored during the inflation (0–60 mmHg). Elastic modulus (E) was also calculated and quantitatively mapped for corneas. The results showed that G′ at 15 mmHg was 86.18 ± 16 kPa and 88.86 + 13.54 kPa inflated by PBS and TC respectively. These values increased 3.2 times in an approximate linear relationship to 60 mmHg. G″ at 15 mmHg was 12.5 ± 2.5 kPa and 13.54 + 1.9 kPa inflated by PBS and TC respectively. G″ increased 1.9 times in an approximate linear relationship to 60 mmHg. No significant change was noticed in viscoelastic properties of corneas inflated by TC for 4 h whereas 4 h of hydration on PBS affected the mechanical properties. The central corneal region was found to be stiffer than in peripheral region. Mapping of elasticity revealed a symmetrical distribution of properties that varied with inflation. Our method has potential for measurement of viscoelastic properties of corneas in conditions where there have been localised changes in mechanical properties such as keratoconus.

Determination of single-crystal elasticity constants of the beta phase in a multiphase tungsten thin film using impulse excitation technique, X-ray diffraction and micro-mechanical modeling

The scope of this work is to propose a methodology allowing the determination of the single-crystal elasticity constants of a phase included in a multiphase thin film taking into account its microstructure (crystallographic and morphological texture, porosity and multiphase aspect). The methodology is based on the use of a macro-mechanical test, the impulse excitation technique, a micro-mechanical test, X-ray diffraction and the Kröner-Eshelby scale transition model. As a supporting example, it was applied to determine the single-crystal elasticity constants of the Wβ tungsten metastable phase embedded in a two phases (α+β) tungsten thin film deposited on a steel substrate by DC magnetron sputtering. The effects of the grain-shape, the crystallographic texture, the porosity and the Wβ volume fraction on the macroscopic elasticity constants were studied. Among all these effects, it was found that the effect of the Wβ volume fraction was the most pronounced. The effects of the crystallographic and morphological texture on the microscopic elastic behavior of the film were evaluated. No dominance of the crystallographic or morphological texture effect was observed and their contributions depend on the crystallographic plane and the measurement direction.

Micromechanical Characterization of Metal Foams

Cellular materials such as foams, honeycombs, or auxetic structures are an interesting type of structural materials for lightweight constructions or dynamic energy absorbers. These microheterogeneous materials consist of a hierarchical structure. Foams can be separated into a macro scale, meso scale, and micro scale dealing with the entire specimen or component, individual pores, and individual struts, respectively. As a result, the macroscopic foam properties are strongly related to the mesoscopic and microscopic properties as a superposition of geometrical structural response and material properties. Whereas the geometry can be easily determined, e.g., by computer tomography measurements, the evaluation of real micro material properties of individual struts or cell walls is still a very challenging task. The material properties of these constituents of open‐cell or closed‐cell foams significantly differ from bulk material properties due to changes in boundary condition during production and different scaling effects, such as different surface‐to‐volume ratios of oxide layers or different cooling conditions due to less material in struts than in the bulk. However, the need for reliable micro material properties for the realistic forecast of macroscopic properties of components made of foam by numerical simulations makes the micromechanical characterization and testing of metal foams to an emerging field of research. The present contribution deals with the most relevant achievements in the field from the beginnings to the present day.

Factorial design approach to assess the effect of fiber–matrix adhesion on the IFSS and work of adhesion of carbon fiber/polysulfone-modified epoxy composites

A two-level full factorial design 22 with three replications was employed to assess the effect of the incorporation of PSF into the epoxy matrix and the surface treatment of carbon fibers on the work of adhesion (WA) and the interfacial shear strength (IFSS) of carbon fiber–epoxy composites. The IFSS was determined using the microbond (or microdrop) micromechanical test, and the work of adhesion was estimated using two different procedures: (1) using the Owens and Wendt method, and (2) from the Dupre–Young expression using the contact angle θ of a cured epoxy resin on a single carbon fiber and the surface energy of the cured epoxy resin. It was found that the treatment of the carbon fiber with the silane-coupling agent appreciably increases its polar component because of the nitric acid oxidation and the chemisorption of the silane-coupling agent onto the carbon fiber surface. Also, the O=S=O group present in the polysulfone chain resin fairly increases the polar component of the epoxy–PSF blend. The results show that the wetting of the silane-treated carbon fiber by the thermoplastic-modified epoxy resin is better, thus increasing the fiber–matrix adhesion. It was also found that there is a similarity between the trends of both, the IFSS and the WA results. Also, from the ANOVA results it was also seen that both the incorporation of the PSF to the epoxy matrix and the surface treatment of the carbon fibers and their interaction were statistically significant to the IFSS and the WA.

Investigation of the fiber-matrix interaction in carbon fiber-reinforced polyether ether ketone by cyclic single fiber push-out and push-back tests

ABSTRACTTo characterize the fiber-matrix interaction of carbon fiber-reinforced polyether ether ketone, a high-performance thermoplastic composite material, micromechanical cyclic single fiber push-out and push-back tests were performed. A new type of successive push-out behavior was observed in addition to the well-known abrupt fiber push-out. To determine the interfacial fracture toughness from the successive push-out load-displacement curves, an advanced energy-based method was developed. It was found that the two different types of push-out correspond to two different values of interfacial fracture toughness. Thermal expansion measurements indicate that residual thermal stress within the investigated sample induces the two different push-out behaviors. The findings suggest that the fiber-matrix interaction might be one reason for the considerable impact of thermal stress on interface dominated macroscopic parameters.

The effects of intergranular carbides on the grain boundary oxidation and cracking in a cold-worked Alloy 600

Intergranular oxidation occurred in a cold-worked Alloy 600 after exposure to pressurized water reactor primary water. Micromechanical testing was conducted to study the process of intergranular cracking under the external loading. High-resolution analytical electron microscopy was conducted to characterize the microstructure and chemistry of the oxidized grain boundaries before and after the micromechanical testing. The matrix around the intergranular carbides was preferentially oxidized and Cr in the intergranular carbides diffused out to facilitate the formation of Cr-rich oxide. Cracks preferentially propagated along the oxide-matrix interfaces during the micromechanical tests. Intergranular carbides are assumed to increase cracking resistance after cold-working.

The effect of B and C addition on microstructure and mechanical properties of TiN hard coatings grown by chemical vapor deposition

Incorporation of additional elements into TiN grown by chemical vapor deposition (CVD) provides an opportunity to enhance the performance of hard coatings used for the protection of cutting tools in the demanding metal processing industry. Within the present work, the hardness, Young's modulus and fracture behavior of CVD coatings within the quaternary system Ti(B,C)N are correlated with their chemical composition and microstructure. TiN, TiBN, TiCN and TiBCN coatings were prepared in an industrial-scale thermal CVD plant by variation of the precursors. X-ray diffraction revealed the majority phase to be face-centered cubic within all investigated coatings, accompanied by small amounts of hexagonal TiB2 in both B-containing coatings and amorphous TiB in TiBN. Transmission electron microscopy unveiled grain refinement and an increment of structural defects, when B, C or a combination thereof is added to TiN. Complementary nanoindentation and micro-mechanical bending tests identified the quaternary TiBCN as the optimum choice with respect to hardness (32.2 ± 1 GPa), Young's modulus (587 ± 29 GPa) and fracture stress (8.5 ± 0.4 GPa). Fracture toughness was superior in the B containing coatings compared to TiN and TiCN with the highest value observed in TiBN (5.0 ± 0.3 MPa m1/2). A post-mortem elevation profile of micro beams' fracture cross-sections verified, that within the B containing coatings the crack is deflected more effectively normal to the crack propagation direction in comparison to TiN and TiCN.